JAK2 inhibitors and their impact in myeloproliferative neoplasms

Abstract

The BCR-ABL-negative myeloproliferative neoplasms (MPNs) include essential thrombocythemia, polycythemia vera, and primary myelofibrosis. Historically, complex biochemical alterations defining these heterogeneously distinct malignancies have remained elusive and constrained available therapy options. The discovery of Janus kinase (JAK) mutations collectively present in BCR-ABL-negative MPNs has led to a resurgence of medical interest in JAK-STAT targeted treatment modalities, as well as provided a unique platform for inhibiting symptom-directing proinflammatory cytokines. INCB018424, CYT387, SB1518, and TG101348 are among the most propitious JAK2 inhibitors under investigation, providing substantial improvement in constitutional symptoms, transfusion-dependent cytopenias, and reduction in spleen size. Despite their attributes, evidence of complete or partial remission has yet to be observed with therapy. Many uncertainties surrounding the full clinical potential of JAK2 inhibitors persist. Treatment guidelines addressing optimal stages for drug implementation, ideal dosing parameters and criteria for medication continuation/withdrawal may effectively resolve these ongoing concerns and provide advancements in the morbidity and mortality of these multifaceted disease processes.

Keywords:

• Essential thrombocythemia
• Polycythemia vera
• Myelofibrosis
• Primary myelofibrosis
• Myeloproliferative neoplasms
• JAK2 inhibitor
• Acute leukemia

Introduction

The 2008 World Health Organization criteria classify myeloproliferative neoplasms (MPNs) to include polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF), chronic myelogenous leukemia, chronic neutrophilic leukemia, and mastocytosis. Historically, the genetic architecture of these stem cell-derived clonal/oligoclonal hemopathies has proven exceptionally intricate, limiting available therapeutic options. The innovative discovery of the BCR-ABL mutation in chronic myelogenous leukemia led to a resurgence of interest in kinase-inhibitor therapies targeting mutant oncoproteins. In 2005, identification of the JAK2V617F mutation in patients with PV, ET, and PMF led to enhanced interest in the development of JAK-STAT targeted treatments.1 According to the World Health Organization, these JAK-2 mutant neoplasms have since been classified as BCR-ABL-negative MPNs.

Prior to this past decade, treatment options for PV and ET focused solely on palliating symptoms and preventing complications intrinsic to the disease. According to current guidelines, low-dose aspirin is indicated in both PV and ET for the prevention of thrombo-hemorrhagic events. In high-risk PV and ET patients, hydroxyurea may be utilized. Hydroxyurea intolerant or resistant patients may be treated with anagrelide, interferon-alpha, or busulfan, depending on patient age. Phlebotomy is the mainstay of treatment in all PV patients with a hematocrit >45%.2 Similarly, treatments for myelofibrosis are not disease-modifying but rather focus on targeting symptoms including hepatosplenomegaly, pancytopenia, and constitutional complaints. Splenomegaly has historically been managed with hydroxyurea with modest results. Splenectomy and splenic/hepatic radiation has also found limited utility given the morbidity associated with post-operative complications. Clinically significant anemia (hemoglobin less than 10 g/dl) is present in up to 54% of patients and has been treated with a variety of agents including thalidomide, lenalidomide, prednisone, and danazol with response rates of roughly 20% and short-lived efficacy.2 Constitutional symptoms including fever, fatigue, bone pain, and night-sweats have historically been resistant to all therapeutic options. The only curative opportunity available, hematopoietic stem cell transplant is restricted to a small percentage of patients.

The 2005 discovery of Janus kinase 2 (JAK2) mutations within the JAK-STAT signaling transduction pathway has opened new opportunities for therapeutic intervention. As a non-receptor tyrosine kinase, JAK2 plays a critical role in signaling the cytokine receptors crucial to the differentiation and ultimate proliferation of hematopoietic progenitor cell lineages including erythrocytes, thrombocytes, leukocytes, and monocytes. Its mechanism includes ligand binding to a receptor leading to phosphorylation of JAK2. This activates downstream transcription factors including STAT3 and STAT5. The mutation JAK2V617F remains the most prevalent cytogenetic abnormality and is present in PV (96%), PMF (65%), and ET (55%). This dominant gain-of-function mutation acquired via a G→T conversion in nucleotide 1849 results in the substitution of valine for phenylalanine. Disruptions within the pseudkinase domain produce a persistently activated molecule as a result of the absence of normal auto-inhibitory effects generated by the pseudokinase domain. In turn, cells harboring the JAK2V617F mutation develop cytokine-independent growth capabilities and/or increased cytokine responsiveness. Other recognized mutations include JAK2 exon 12, MPL exon 10, LNK (SH2B3), and LNK exon 2 and are present to a lesser extent in BCR-ABL-negative MPNs. Notably, recent evidence suggests that mutations inherent to the JAK-STAT pathway do not necessarily induce disease initiation but rather define the MPN subclone that corroborates with other mutations, ultimately creating the disease phenotype. Of further interest, increased JAK2V617F allele burden is not associated with increased thrombosis risk, leukemic transformation, or mortality. In conjunction, both poor survival and higher risk of acute leukemia have been associated with low allele burden.3

JAK2 Inhibitor Treatments

INCB018424 (ruxolitinib)

The oral JAK1/JAK2 inhibitor, ruxolitinib (INCB018424), has emerged as a promising JAK kinase modulator with notable efficacy in BCR-ABL-negative MPNs. It remains the most advanced JAK2 inhibitor in development. Though still under investigation, preclinical studies have shown effectiveness in reducing the pro-inflammatory cascades mediated by JAK1 as well as mitigating JAK2-mediated extramedullary hematopoiesis. Recent data suggest that utility is displayed regardless of JAK2 mutation status rendering this drug active independent of the presence of the JAK2V617F mutation. The primary dose limiting toxicity is thrombocytopenia, which appears reversible. Other symptoms include dizziness (2·6%), nausea (2%), headache (3·3%), and diarrhea (5·9%). A phase I/II trial published in 2010 studied the effectiveness of INC018424 in primary and secondary MF patients (Table 1).4 Most patients experienced a 50% or greater improvement in symptoms based on the Myelofibrosis Symptom Assessment Form with an overall response rate averaging 71–78%. Improvements included enhanced exercise capacity, decreased pruritis, and weight gain. Notably, 44% of patients with splenomegaly had a reduction of >50% within 3 months of therapy. Roughly 14% of patients previously red cell transfusion-dependent at study initiation experienced clinical improvement with transfusion independence for greater than 20 months. Biochemical analysis identified successful pharmacological inhibition of the JAK-STAT pathway after 28 days of treatment as exhibited by reduction in p-STAT3 levels to those similar of controls. The maximum tolerated dose was 25 mg b.i.d or 100 mg q.d. Significant adverse effects included thrombocytopenia at rates of 23% and 10% at the 25 mg b.i.d. and 10 mg b.i.d. dosing levels. Anemia occurred at rates of 27% and 16% at the 25 mg b.i.d. and 10 mg b.i.d. dosing levels, respectively.

Table 1. JAK 2 inhibitors in development for myeloproliferative neoplasms

Drug	Disease entity	Kinase target	Primary efficacy	Prominent side-effects	References
INCB018424 (ruxolitinib)	PV, ET, MF	JAK1, JAK2	Splenomegaly	Anemia, thrombocytopenia	5,6
CYT387	MF	JAK1, JAK2, TYK2, TBK1, PRKD1, ROCK2, PRKCN, MAPK8, and CDK2/cyclin A	Splenomegaly, anemia, constitutional symptoms	Thrombocytopenia	7
SB1518	MF	JAK2, FLT3	Splenomegaly, constitutional symptoms	Diarrhea	8
TG101348	MF	JAK2, RET, FLT3	Splenomegaly, thrombocytosis, leukocytosis, constitutional symptoms	Anemia, thrombocytopenia, nausea, vomiting, diarrhea	9
CEP-701	PV, ET, MF	JAK2, FLT3, VEGFR, TRKA	Splenomegaly	Anemia, thrombocytopenia, diarrhea, nausea, vomiting	10

Note: ET, essential thrombocytopenia; PV, polycythemia vera; MF, myelofibrosis.

The Controlled Myelofibrosis Study with Oral JAK Inhibitor Treatment trials (COMFORT-1 Trial and COMFORT-2 Trial) are two investigational studies evaluating INCB018424 in primary or secondary MF presented in abstract form at the 2011 American Society of Clinical Oncology meeting. COMFORT-1 is a randomized, double-blind, placebo-controlled phase III study with 309 intermediate-2/high risk patients assigned to twice-daily oral INCB018424 (15 or 20 mg) or placebo. The primary endpoint was a reduction in spleen volume by 35% by the end of week 24 of therapy. Patients randomized to the INCB018424 arm were statistically more likely to attain the primary endpoint (41·9%) versus placebo (0·7%). Patients were also more likely to have symptomatic improvement in abdominal discomfort, early satiety, itching, bone or muscle pain, pain under left ribs, night-sweats, and inactivity. Anemia (45·2%) and thrombocytopenia (12·9%) were the two grade 3/4 adverse events observed more frequently in comparison to placebo.

COMFORT-II is a randomized, open-label phase III study of INCB018424 in comparison to best available therapy (BAT) in patients with intermediate-2/high risk primary or secondary MF (Table 1).5 The primary endpoint was evaluation of the proportion of subjects achieving >35% reduction in spleen volume from baseline to week 48. The week 48 response rate was 28·5% for INCB018424 patients versus 0% for BAT. Beginning at week 8, highly significant improvement was noted in spleen volume for patients receiving INCB018424. Thrombocytopenia (44·5%), anemia (40·4%), diarrhea (24·0%), and peripheral edema (21·9%) remained the most common adverse effects. Thrombocytopenia and anemia were the most common toxicity limiting side-effects. Pneumonia, dyspnea, thrombocytopenia, and anemia were the most frequent grade 3/4 adverse events.

Currently ongoing, the RESPONSE Trial is a phase III, open-label trial designed to compare BAT to INCB018424 in hydroxyurea-resistant PV patients requiring phlebotomy.6 Enrolled patients must require phlebotomy due to inadequate hematrocrit and have palpable splenomegaly (>5 cm below coastal margin) with either leukocytosis (>15×10⁹) or thombocytosis (>600×10⁹). The primary endpoint will include both phlebotomy independence and a 35% reduction in spleen volume after 32 weeks of treatment. Enrollment is now open globally.

CYT387

CYT387 non-selectively inhibits a variety of kinases including JAK1/2, TYK2, TBK1, PRKD1, ROCK2, PRKCN, MAPK8, and CDK2/cyclin A. An abstract presented at the 2010 ASH Conference evaluated 36 primary or secondary MF patients. Patients were initiated on either 150 or 300 mg/day. Total anemia response rate was 63%, including two patients who had previously received INCB018424. Splenomegaly improvement was identified in 97% of patients, with 37% obtaining at least a 50% decrease in size. Notably, 38% of all splenomegaly responders had previously received INCB018424. Constitutional symptoms including fatigue, nigh-sweats, fever, bone pain, and pruritis underwent significant clinical improvement. Grade 3 non-hematologic adverse toxicities were rare and included headache (n = 2), QTc prolongation (n = 1), and abnormal liver/pancreatic tests (n = 5). Hematologic toxicities included grade 3/4 thrombocytopenia (22%) and grade 3 anemia (3%).

Presented at the 2011 American Society of Clinical Oncology Conference, an ongoing phase 1/2 multi-center study is currently evaluating CYT387 in 108 high/intermediate risk patients with MF (Table 1).7 Daily oral administration occurred in 28-day cycles. The maximum tolerated dose was 300 mg/day. A transient first-dose effect was observed in roughly 50% of patients and typically resolved within hours. Grade 3/4 toxicity was rare with the exception of thrombocytopenia (25%). Overall, the anemia response rate was 58% in transfusion-dependent patients with a median duration of anemia response of 20 weeks. Constitutional symptoms including bone pain, pruritis, and night-sweats resolved in the majority of patients. Spleen-reduction response rate was 45%. A phase II multi-center, open label, non-randomized dose-escalation/dose-confirmation study is currently enrolling patients.

SB1518

SB1518 has emerged as a selective inhibitor of JAK2V617F as well as wild-type JAK2 and FLT3. In a recent phase 1 study, all doses of SB1518 (100–600 mg/day) achieved steady-state plasma levels above the IC50 and resulted in target inhibition measured by phosphorylation of STAT3, STAT5, and JAK proteins. An abstract presented at the 2011 American Society of Hematology (ASH) Conference evaluated 33 patients with primary or secondary MF. Patients were prescribed 400 mg daily in a 28-day cycle (Table 1).8 More than 96% of patients experienced significant reduction in spleen volume; 57% had a reduction greater than 25% from baseline by magnetic resonance imaging. In addition, there was a 40–65% reduction in the intensity in MF-related symptoms in patients receiving treatment for 6 months. Significant neutropenia and thrombocytopenia were not identified. The only grade 3 non-hematologic symptom was diarrhea. Other adverse events included diarrhea (81%), nausea (41%), vomiting (22%), and fatigue (9%). Hematologic adverse events were minimal.

TG101348

TG101348 is a JAK2 inhibitor with documented activity against other kinases including RET and FLT3. A phase 1 study in 2011 evaluated 59 high/intermediate-risk primary and secondary PMF patients (Table 1).9 The maximum tolerated dose was 680 mg/day. Within 12 months, 47% of patients achieved a significant decrease in spleen size, with a mean response duration of 315 days. More than 50% of patients had resolution of cough, fatigue, pruritis, early satiety, and night-sweats. Thrombocytosis and leukocytosis were normalized in 90 and 57% of patients, respectively. At 6 months, a significant reduction in the JAK2V617F allele burden was identified; this persisted at 12 months. The most common grade 3/4 toxicities included nausea (3%), vomiting (3%), diarrhea (10%), anemia (35%), and thrombocytopenia (24%). The dose limiting toxicity was an asymptomatic and reversible increase in the serum amylase level.

CEP-701

CEP-701 is a kinase inhibitor with activity against both JAK2V617F and wild-type JAK2, as well as FLT3. Previously finding efficacy in AML patients, a phase 2 study of CEP-701 in 22 MF patients was recently completed (Table 1).10 Patients were scheduled to receive 80 mg/BID. The overall response rate was 27% with a median time to response of 3 months. Only three patients experienced a decrease in spleen size >50%. Two patients achieved transfusion independency (9%). Dose reduction was required by 27% of patients for grade 3/4 toxicity. Hematologic side effects included significant anemia and thrombocytopenia experienced at 14 and 23%, respectively. Non-hematologic toxicity included diarrhea (72%), nausea (50%), and vomiting (27%). At completion of the first stage of the trial, a risk/benefit analysis did not justify study continuation.

Summary

Despite significant advances in treatment options available for BCR-ABL-negative MPNs, many outstanding questions warrant further review. Optimal dosing strategies are an area of continued debate as escalated dosages often result in improved efficacy with intolerable side-effect profiles. Alternative techniques such as weight-based dosing schedules or allele-burden determined dosing have yet to be investigated. Others have suggested an ‘induction-maintenance approach’ which would utilize higher doses until pre-determined criteria are met, followed by a lower maintenance dose. The ideal disease stage to start treatment has also yet to be addressed. Most JAK2 inhibitor clinical trials have involved MPN patients with advanced or progressive disease burden. The efficacy of JAK2 inhibitors in earlier stages of myelofibrosis has not yet been investigated. Specific criteria for discontinuation have also not yet been established. Previous studies have utilized drug intolerance or trial completion as grounds for discontinuation. However, withdrawal of JAK inhibitors predictably results in splenic growth, symptomatic relapse and at times, hemodynamic instability. With long-term safety still under investigation, indefinite continuation should be approached with caution. It has been suggested that slow tapering over a 2–4 week period may mitigate a ‘cytokine-flare’ that occurs with abrupt withdrawal.

Conclusion

The development of JAK targeted treatment options has provided new avenues for therapeutic intervention in mitigating the debilitating disease effects of BCR-ABL-negative MPNs. INCB018424 (ruxolitinib), CYT387, SB1518, and TG101348 have evolved as the most promising JAK2 inhibitors among the furthest developed compounds in regards to their side-effect profiles, effectiveness and demonstrable efficacy against pro-inflammatory cytokines. As we continue to acquire data through future investigational efforts, it remains to be determined if the same benefits in morbidity are observed in mortality. It is our hopes that continued efforts in this field will ultimately address the many questions inherent to this new therapeutic modality.